Sticky Chemistry: Intermolecular Forces

Intermolecular forces (forces between chemical species) are important in biochemistry. Among other things, intermolecular forces are important to hydrophilic (water-loving) and hydrophobic (water-hating) interactions.

All intermolecular forces are van der Waals forces; that is, they are not true bonds in the sense of sharing or transferring electrons, but are weaker attractive forces. These forces include dipole-dipole forces, hydrogen bonding, and ionic interactions.

Dipole-dipole forces exist between polar regions of different molecules. The presence of a dipole means that the molecule has a partially positive end and a partially negative end. Opposite partial charges attract each other, whereas like partial charges repel. In most cases, biological systems utilize a special type of dipole-dipole force known as hydrogen bonding.

Hydrogen bonding, as the name implies, involves hydrogen. The hydrogen atom must be bonded to either an oxygen atom or a nitrogen atom. Hydrogen bonding is significantly stronger than a "normal" dipole-dipole force and is very much stronger than London dispersion forces (very weak and short-lived attractions between molecules that arise due to the nucleus of one atom attracting the electron cloud of another atom). Hydrogen bonding may be either intramolecular or intermolecular.

Ionic interactions may serve as intermolecular or intramolecular forces in biological systems. In some cases, these may involve metal cations, such as Na+, or anions, such as Cl-. In many cases, the cation is an ammonium ion from an amino group; the anion may be from a carboxylic acid. Oppositely charged ions attract each other strongly.

Water-related interactions

The predominant factor leading to hydrophobic (water-hating) interactions is the presence of portions of a molecule containing only carbon and hydrogen. Hydrocarbon regions are nonpolar and are attracted to other nonpolar regions by London dispersion forces.

In general, the presence of any atom other than carbon and hydrogen makes a region polar. Oxygen and nitrogen are the most effective elements in biochemistry for making a region of a molecule polar. Sulfur is least effective of the common biologically important elements at imparting polar character. Dipole-dipole, hydrogen bonding, and ionic interactions are all hydrophilic (water-loving) interactions. London dispersion forces are hydrophobic interactions.

How bond strengths affect physical properties of substances

The physical properties of biological substances depend on the intermolecular forces present. The sequence of strength from strongest to weakest force is ions > hydrogen bonding > dipole-dipole > London forces. As the strength of forces decreases, so do the melting points, boiling points, and solubility in water. Besides that, the vapor pressure and the solubility in nonpolar solvents also increases.

A molecule may have both hydrophilic and hydrophobic regions. The region that represents a greater portion of the molecule will predominate.

Many functional groups have distinctive odors. Small carboxylic acids smell like acetic acid (vinegar), while larger ones have unpleasant odors. Most esters, if volatile, have pleasant odors — that is why esters are used extensively in the flavor and perfume industry. Most sulfur-containing compounds have strong unpleasant odors. Small amines have an ammonia odor, whereas larger amines have a fishy odor or worse.